1. Field of the Invention
This invention relates to optical interference coatings for lamps and finds particular application in conjunction with a multi-layer ultraviolet (UV) reflecting coating for an arctube of a metal halide lamp which reflects UV radiation into the metal halide pool for increased efficiency of the lamp.
2. Discussion of the Art
Metal halide lamps use a fill comprising a metal halide, mercury, and a rare gas. The metal halides, which often comprise sodium iodide and scandium iodide, are partially vaporized from the molten liquid pool during lamp operation. When the lamp is energized, an arc discharge is created, which emits radiation at wavelengths above about 200 nm. Radiation emitted between 100-400 nm is ultraviolet (UV) radiation, which is harmful to human eyes and skin and which also causes fading, discoloration and degradation of fabrics, plastics, and paints. In addition to the harmful effects of UV radiation which escapes a lamp, the UV radiation is essentially wasted, since it does not contribute to useful, visible illumination.
In a typical operating quartz metal halide (QMH) lamp, the molten metal halide pool absorbs at least 50% of the incident radiation below 450 nm, and at least 80% of that below 400 nm. In vertical operation of a cylindrically shaped arctube, the molten metal halide pool may cover the lower 25% of the inside surface of the arctube. As the temperature of the metal halide pool is increased, the vapor pressure of the metal halide gas is increased, and the relative contribution of the metal halide to the fill is enhanced, relative to the contribution from the mercury. This imparts the metal halide lamp with higher efficacy and better color, relative to a high-pressure mercury lamp.
In some QMH lamps, a heat-conserving endcoat, often comprising alumina, is applied to the outside surface of the arctube over the area corresponding to the metal halide pool in order to operate the metal halide pool at a higher temperature. In some vertically-burning QMH designs, the lower end of the arctube is made with a smaller diameter than the upper end to heat the metal halide pool more efficiently, without overheating the upper end of the arctube.
Several methods have been employed to block the emission of the harmful UV. Traditional metal halide lamps employ an arctube of generally pure quartz enclosed in a glass outer jacket to provide UV protection. Newer metal halide products, however, such as fiber optic sources (see, for example, U.S. Pat. No. 4,958,263) and automotive lamps (see, for example, U.S. Pat. No. 4,868,458), encounter size and use constraints which limit the use of glass outer jackets. Arctubes made from doped quartz (see, for example, U.S. Pat. No. 5,196,759) have been used to absorb the UV emissions from the arc within the arctube itself, thus eliminating the need for a glass outer jacket. However, the UV-absorbing dopants in doped quartz cause enhanced devitrification and shortened lamp life. Naturally occurring “hot spots” within the arctube wall are formed directly above the discharge in metal halide arctubes due to natural convection of the gases inside the arctube. Absorption of UV radiation by a doped quartz arctube tends to worsen further the hot spots on the arctube wall. The greater susceptibility of doped quartz to devitrification and softening, aggravated by the additional overheating at the hot spot due to absorption of UV by the doped quartz, can result in rapid failure of the lamp. For this reason, it is common to restrict the use of doped quartz to shrouds or jackets surrounding the arctube, rather than using it for the arctube, itself.
A further option for UV protection is the use of coatings on the outer surface of the arctube. U.S. Pat. No. 5,336,969 describes the use of a coating comprising a suspension of CeF3 and Al2O3.SiO2, which is applied to the arctube surface and fired. This coating operates to absorb UV radiation. U.S. Pat. No. 4,949,005 discloses a tantala-silica interference filter used on a tungsten halogen lamp to reflect or absorb specific wavelengths by variation in filter design. IR heat radiation is preferentially reflected back to a tungsten filament to improve the efficacy of the halogen lamp. U.S. Pat. No. 5,552,671 discloses a UV reflecting multi-layer coating for use on a metal halide arctube, which absorbs deep UV and reflects near UV radiation. The reflective nature of the coating allows UV radiation to bounce off the coating and be absorbed by the metal halide pool. This allows attenuation of the UV emission from the arctube, without the use of doped quartz with its attendant overheating of the quartz hot spots, in order to increase the life of the arctube.
The present invention provides an optimized UV reflecting multi-layer coating and method of preparation, which overcome the above-referenced problems and others.